I'm reading through this one story where it takes place in the feudal era of Japan. Two men are travelling through a forest and carrying a secret letter. An army of female ninja try to sneak up on them. But one of the men can smell them, so he closes his eyes and concentrates. Detecting the location one of them, he throws his sword like an arrow, and hits her. Then he readies himself for the rest of them.

At that point, the narration says “どしゅっ!” I'm guessing that this is an interjection of some sort. I tried to look it up, but it doesn't seem to be listed in the dictionaries. There are websites featuring it, but they don't have any explanations for it in Japanese or any other language.

I can't thank you enough! By the way, in the same story, one character gets bisected. The sound effect for this is どしゅんっ! Is this similar to どしゅっ?
– Micheal GignacJun 6 '18 at 0:14

Right, authors often "create" their own versions of onomatopoeias, but the new versions will usually end up sounding/looking like the prototype. That is because otherwise, the readers/viewers would not recognize them.
– l'électeurJun 6 '18 at 6:18